1. Field of the Invention
The present invention relates generally to a handoff method in a mobile communication system, and in particular, to a handoff method in a High-Rate Packet Data (HRPD) mobile communication system.
2. Description of the Related Art
In general, a mobile communication system transmits and receives information over a radio channel in order to provide mobility to an access terminal. Such a mobile communication system has evolved from a system that basically supports a voice service into a system that can transmit and receive packet data. For the system that can transmit and receive packet data, a system that transmits simple short messages and small amounts of Internet service data is currently used. In addition, in order to meet users' demands for data service, the mobile communication system has evolved into a High-Rate Packet Data (HRPD) mobile communication system, called a 1x EV-DO mobile communication system. Currently, the 1x EV-DO mobile communication system is being used.
With reference to FIG. 1, a description will now be made of configuration and operation of a conventional HRPD mobile communication system. FIG. 1 is a diagram illustrating configuration of an HRPD mobile communication system.
An access terminal (AT) 101 can perform data communication with an access network transceiver system (ANTS) over a radio channel. Access network controllers (ANCs) 110 and 120 are connected to a plurality of ANTSs 11 to 1N and 21 to 2N, respectively, and control their associated ANTSs. The ANC can constitute a subnet independently or as one access network (AN), or two or more ANCs can constitute a subnet as one AN. Such an AN-based subnet is identified through a unique subnet mask prepared in the ANC. Since the subnet mask is comprised of 128 bits, the subnet can also be identified through a color code comprised of fewer bits. The ANCs 110 and 120 have session controllers 111 and 121, respectively. Each of the session controllers 111 and 121 assigns a unicast access terminal identifier (UATI) to an AT located in an ANC connected thereto, and manages session information corresponding to the AT.
The ANCs 110 and 120 can be connected to a data core network (DCN) 150 directly or via another specific ANC. The DCN 150 is connected to the ANC via a packet data service node (PDSN) 151. A detailed description of the DCN 150 will not be given. The ANCs 110 and 120 can be connected to an access network-authentication, accounting and authorization (AN-AAA) system 160 independently or indirectly via a specific AN-AAA system.
With reference to FIG. 2, a description will now be made of a handoff method in such an HRPD mobile communication system. FIG. 2 is a signaling diagram during handoff of an AT in an HRPD mobile communication system.
In an HRPD mobile communication system, session information of subscribers is stored in the session controller 111. Therefore, handoff occurs when session information is turned over. That is, handoff takes place when session information of an AT is transferred to a session controller of another ANC, and in this way, history of the AT is continuously maintained. Such a session-related handoff standard is defined as A13 in IS-878, a 1x EV-DV standard. Such a handoff procedure will now be described with reference to FIG. 2.
In FIG. 2, the ANC 120 to which the AT 101 first belongs is called a source ANC, and a session controller (SC) 121 connected to the source ANC 120 is called a source SC. In addition, the ANC 110 to which the AT 101 is to move is called a target ANC, and the session controller 111 connected to the target ANC 110 is called a target SC. The signaling procedure of FIG. 2 will now be described.
The AT 101 can detect a change of ANC through check of a subnet mask or check of a color code. Upon detecting the change of ANC, the AT 101 transmits a UATI Request signal UATIRequest to the target ANC 110 in step 200. At this point, the AT 101 transmits a subnet mask or a color code of an ANC to which it previously belonged, together with an old UATI. Upon receiving the UATI Request signal, the target ANC 110 transmits in step 202 a UATI Allocate Request signal UATIAllocateReq to the target SC 111 along with the subnet mask or color code and the UATI. The target SS 111 can identify the source SC 121 using the received subnet mask or color code. After identifying the source SC 121, the target SC 111 transmits in step 204 an A13 Session Information Report message based on an A13 standard to the source SC 121.
Upon receiving the A13 Session Information Report message, the source SC 121 transmits in step 206 an A13 Session Information Response message to the target SC 111 along with session information for a call of a corresponding AT that received the information, and subscriber information. Such an A13 Session Information Response message includes a message sequence value. If the A13 Session Information Response message is received in step 206, the target SC 111 allocates again a UATI for the AT 101 in step 208. Further, in step 208, the target SC 111 generates the newly allocated UATI as a UATI Allocate Response signal UATIAllocateRsp and sends the generated UATI Allocate Response signal to the target ANC 110. At this point, if IP Address and Access Network Identifier parameters of a PDSN are received from the source SC 121, the target SC 111 sends the UATI Allocate Response signal UATIAllocateRsp to the target ANC 110 along with the received parameters. Because a UATI of the AT 101 was newly assigned, the target ANC 110 sends in step 210 a UATI Assignment signal UATIAssignment to the AT 101 to inform the AT 101 of the newly assigned UATI. If a UATI is newly assigned in this way, the AT 101 stores the newly assigned UATI. Thereafter, in step 212, the AT 101 sends a UATI Complete signal UATIComplete to the target ANC 110 to indicate that the UATI has been correctly assigned.
Upon receiving the UATI Complete signal UATIComplete, the target ANC 110 generates in step 214 a UATI Complete Request signal UATICompleteReq and sends the generated UATI Complete Request signal to the target SC 111. Upon receiving the UATI Complete Request signal, the target SC 111 generates in step 216 a UATI Complete Response signal UATICompleteRsp and sends the generated UATI Complete Response signal to the target ANC 110. Then the target ANC 110 and the AT 101 perform a Location Update procedure in step 218. Further, in step 220, the target SC 111 generates an A13 Session Information Confirm signal according to an A13 standard and sends the generated A13 Session Information Confirm signal to the source SC 121. Upon receiving the A13 Session Information Confirm signal, the source SC 121 deletes a database (DB) for the corresponding AT.
As described above, many signaling procedures are performed while the AT 101 requests a UATI upon detecting movement of its position and then is assigned a UATI in response to the request. In addition, even after the UATI is assigned, signals must be exchanged between the target ANC 110 and the AT 101 during the Location Update procedure, and signals should also be exchanged between the session controllers.
However, in some cases, an AT fails to receive pilot signals from a target ANC and a source ANC while performing handoff. Such a case takes place when the AT fails to receive a pilot signal as the AT is powered off or a channel condition becomes poor during handoff. When the AT fails to acquire a pilot signal received from the source ANC and the target ANC during handoff like this, the AT fails in handoff.
In case of the handoff failure, the AT must perform a process related to initial location registration when it acquires again a pilot signal. That is, the AT must restart session negotiation performed during the initial location registration from the beginning. In this case, even if only 3 protocols of ‘session configuration protocol’, ‘stream protocol’ and ‘idle state protocol’ are assumed as attribute protocols performed for session negotiation, the AT must newly perform negotiation and determine available protocols at each session. Even if only 3 protocols are assumed like this, when session negotiation is newly performed, the number of messages exchanged between an AT and an access network (AN) in which an ANTS is included must be 7. This is because each message requires a configuration request message and its associated configuration response message, and when the negotiation is completed, it must include a configuration complete message.
That is, in case of handoff failure, so many messages must be exchanged increasing a system load. As a result, available resources in the system are exhausted. In addition, when so many messages are exchanged like when the AT is initially powered on, a user may feel tedious resulting in deterioration in service quality.